Colin Firth has joined Reese Witherspoon in Atom Egoyan-directed drama about the West Memphis 3, Devil's Knot.

The actor will play Ron Lax, a private investigator who was the first pro bono supporter of the defendents as they headed to trial in 1993. Lax built one of the most prominent private investigative firms in the Southeast, and offered his services for free to the defendants, who at the time were reviled because of the heinous nature of the crime and the sensationalized reports about devil worship and ritualistic sacrifice. All of that was later proven to be unfounded, after Damien Echols, Jason Baldwin and Jessie Misskelley were convicted despite not a shred of physical evidence. The detective was an unsung hero and helped find the DNA in the knots that bound one of the victims, that cast suspicion on Terry Hobbs, the stepfather of Steven Branch, one of the 8-year old murdered boys.

Reese Witherspoon will play Pam Hobbs, the mother of Branch. She initially believed the trio murdered her son, is eventually persuaded that the three suspects are innocent and wrongly accused.

The script is based on investigative reporter Mara Leveritt's 2003 book Devil's Knot: The True Story of the West Memphis Three. The book is an in-depth chronicle of the trials that sent the three to prison for the murder of the three 8-year old boys who were found hog-tied in a drainage ditch.

The project begins filming next summer in Louisiana.